Aerodynamics of Bluff-Body Stabilized Confined Turbulent Premixed Flames

Abstract

Detailed information on the influence of geometric and flow parameters on the structure and properties of recirculation zone in confined combusting flows is not available. In this paper, recirculation zone structure and turbulence properties of methane-air mixtures downstream of several conical flameholders were measured using LDA. These tests employed different blockage ratios (13 and 25 percent), cone angles (30, 45, 60, and 90 deg), equivalence ratios (0.56, 0.65, 0.8, and 0.9), mean annular velocities (10, 15, and 20 m/s), and approach turbulence levels (2, 17, and 22 percent). It was found that increasing the blockage ratio and cone angle affected the recirculation zone size and shape only slightly. Also, these parameters increased the shear stress and turbulent kinetic energy (TKE) moderately. Increasing the equivalence ratio or approach turbulence intensity produced a recirculation zone shape very similar to that found in the cold flow. TKE decreased due to turbulent dilatation produced by increased heat release. These observations are discussed from the viewpoint of their importance to practical design and combustion modeling.

Lean blowoff of bluff body stabilized flames: Scaling and dynamics

“…There is a wide range of data in the literature showing L recirc increasing, decreasing, or staying constant with changes in fuel/air ratio, bluff body type, and fuel/air ratio [17,74,75]. Two different perspectives of the recirculation zone are presented in Fig. 14.…”

“…This is apparently due to the stabilization of the BVK instability, as discussed in the next section, as well as the substantial increase in viscosity in the post-flame gases and the gas expansion induced vorticity sink. Other computations and data on the structure of the mean and fluctuating velocity field are presented in Beer and Chigier [14], Pan et al [17], Fureby and Moller [58], Fuji and Eguchi [56], Bill and Tarabanis [57], and Yang et al [59].…”

“…It is helpful to categorize the shear layer momentum transport processes by whether they are molecular diffusivity, transitional, or turbulence dominated. In the turbulent regime, transverse turbulent momentum transport increases proportional to flow velocity, leading to L recirc scaling that is invariant with flow velocity/Reynolds number [17,[74][75][76]. For an analogous reason, L recirc increases with approach flow velocity in the laminar and transitional regimes.…”

“…Discussion of the fluid mechanics of axisymmetric bluff bodies can be found in Achenbach [15]. In addition, Zukoski's thesis [16] discusses the differences between the two-dimensional and axisymmetric bluff body flows and Pan et al [17] provides extensive measurements of the reacting flow field. The second topic is bluff body stabilized non-premixed flames, such as occurs in cases where fuel is injected into the bluff body wake (not to be confused with premixed flames with spatially varying stoichiometry) [18,19].…”

Impact of fuel composition on the recirculation zone structure and its role in lean premixed flame anchoring

“…In a trapped vortex combustor, for instance, the streamwise length of the RZ is bound by the geometry of the cavity. In all other configurations, the RZ "floats," where its dimensions exhibit strong sensitivities to flow parameters (e.g., velocity, Reynolds number, turbulent intensities), geometrical features (e.g., blockage, corner sharpness) and combustion related parameters such as equivalence ratio and preheat temperature [4,5,6]. …”